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OCR for page 48

48
=
(a) Experiment (Hjorth,1975)
=3
5
7 3
9
11 1
Flow
-1 -0.5 0 0.5
X/B
(b) Numerical
Figure 6.2. Comparison of bed shear stress distribution (N/m2) around a
circular pier as calculated from an experiment by Hjorth (1975) and
numerical computations (B = 0.075 m, V = 0.30 m/s, H = 0.2 m).
By regression, the equation proposed for the correction 6.7 PIER SPACING EFFECT:
factor kw giving the influence of the water depth on the max- NUMERICAL SIMULATION RESULTS
imum shear stress is
The objective of this parametric study is to obtain the rela-
max -
4H
tionship between the maximum bed shear stress max and pier
kw = = 1 + 16e B (6.2)
max (deep) spacing (Figure 6.11).
One of the flume experiments was chosen to perform the
numerical simulation. The cylindrical pier had a diameter
of 0.16 m and was placed vertically in a 1.5-m-wide flume.
The mean depth approach velocity was 0.33 m/s and the water
depth is 0.375 m. Four different pier spacings were simu-
H lated: S/ B = 6 (in the case of one pile in the flume), S/ B =
Flow 3.12 (in the case of two piles), S/ B = 2.34 (in the case of
three piles), and S/ B = 1.88 (in the case of four piles). The
Reynolds Number based on diameter was Re = 52800 and
the Froude Number based on diameter was Fr = 0.2634. The
B
velocity between the piles became higher due to the de-
Figure 6.3. Problem definition for water depth effect. creased spacing and the corresponding shear stress increases.